The present invention relates to an apparatus for controlling a motor, and particularly to an apparatus for controlling a brushless sensorless motor.
In order to reduce battery power consumption and lengthen continuous reproduction time of a portable MD (minidisk) system powered by a battery, for example, a motor for rotating an MD is controlled to rotate at high speed for fast access only when the MD is accessed to make recording and reproduction, and is otherwise controlled to rotate at low speed to reduce power consumption.
FIG. 6 is a waveform diagram showing change in drive voltage when rotational speed of the motor is to be changed. FIG. 7 is a waveform diagram showing change in the rotational speed of the motor when the drive voltage is changed in a manner as shown in FIG. 6. In this example, until a time T1, it is not necessary to access the MD, and therefore the drive voltage is held low and also the rotational speed is low. During a period from the time T1 to a time T2, the drive voltage is set high in order to access the MD, and thus the rotational speed is also high. After the time T2, the drive voltage is zero, and the motor rotates by inertia and gradually lowers its rotational speed.
It is advantageous from a viewpoint of cost and the like to use a brushless sensorless motor, which eliminates the need for a sensor such as a Hall device to detect the rotational speed, as the motor for driving and rotating the MD. The motor includes three coils, which form a three-phase coil, and on the basis of a pulse signal generated from one of the coils, an FG pulse signal that represents the rotational speed of the motor by frequency is generated.
However, in a case where the rotational speed of the brushless sensorless motor is changed to lengthen continuous reproduction time as described above, when the rotational speed is greatly changed in a short time, the FG pulse signal is not properly generated, and thereby servo control cannot be effected. This may render control of the motor impossible.
The present invention has been made to solve such problems, and it is accordingly an object of the present invention to provide a motor control apparatus that enables the brushless sensorless motor to operate stably even when its rotational speed is changed.
In order to achieve the above object, according to the present invention, there is provided a motor control apparatus comprising: a servo circuit for outputting a pulse width modulation signal; a low-pass filter for receiving the pulse width modulation signal outputted by the servo circuit as an input; a driver circuit for receiving an output voltage of the low-pass filter as an input and thereby driving a brushless sensorless motor; and rotational speed changing means for changing rotational speed of the motor by gradually varying a voltage outputted by the driver circuit from a current voltage to a target voltage on input of a control start signal; wherein the servo circuit controls pulse width of the pulse width modulation signal so as to maintain the rotational speed of the motor at a target rotational speed on the basis of an FG pulse signal outputted from the motor, the FG pulse signal representing the rotational speed of the motor by frequency.
With this configuration, on input of the control start signal to change the rotational speed of the motor, the rotational speed changing means in the motor control apparatus according to the present invention changes the rotational speed of the motor by gradually varying the voltage outputted by the driver circuit from the current voltage to the target voltage. Thus, the rotational speed of the motor also changes gradually, and the motor properly generates the FG pulse signal. Therefore, even when the rotational speed is varied greatly, it is possible to properly control the motor on the basis of the FG pulse signal.
The above and other objects, features and advantages of the present invention will become apparent from the following description and the appended claims, taken in conjunction with the accompanying drawings in which like parts or elements denoted by like reference symbols.